The invention discloses a weak fault identification and performance degradation monitoring method of a complex epicyclic gear train. The method comprises the following steps: constructing a standard multi-wavelet basis function with symmetric orthogonal multi-wavelet construction conditions as constraints; selecting optimal multi-wavelet matched with a to-be-monitored fault signal according to the standard multi-wavelet basis function; extracting and identifying a weak fault of a dynamic signal of the complex epicyclic gear train according to the optimal multi-wavelet; and performing health state and performance degradation monitoring on the complex epicyclic gear train via a feature fusion index. By adoption of the weak fault identification and performance degradation monitoring method disclosed by the invention, the purpose of identifying the weak fault of the complex epicyclic gear train is realized, the mechanical fault position, category and damage degree are accurately disclosed, and the gradient trend of the reliability of the complex epicyclic gear train is effectively disclosed.